1. Field of the Invention
The invention relates in general to an adaptive frequency range equalizer for digital radio relay systems in which an intermediate frequency position has mounted therein a transversal filter with adjustable coefficients and has means for analyzing the spectrum for the setting of the coefficients.
2. Description of the Prior Art
Basic concepts of adaptive frequency range equalizers for digital radio relay systems is disclosed in the publication Proceedings ICC'85 No. 39.1.1-39.1.5 which reports related problems in the article entitled "Design and Performance of an Adaptive IF Equalizer For 140 Nbits/s16 QAM System".
In digital radio relay systems, particularly when multi-stage quadrature amplitude modulation is used, a preliminary equalization is carried out of the power density spectrum of the received signal which has experienced linear distortions as a result of multi-path propagation. Such frequency range equalization can be optimized by using time range equalizers and combination. Particular consideration must be given to the so-called "non-minimum phase" channel situation which cannot be equalized by base-band equalizers with quantitized feedback (decision feedback) with the same level efficiency as the "minimum-phase" channel transmission function.
The previously known frequency range equalizers which operate in the intermediate frequency position obtain the information required for setting them up from filter probes in the IF position (spectrum analysis filter bank) and utilize minimum phase networks composed of concentrated elements to carry out the equalization.
Generally so-called "slope equalization", "square equalization" or "moving notch equalization" functions are performed all of which exhibit a considerably higher level of efficiency in the minimum phase than in the non-minimum phase situation.
Also, the danger exists in such equalizer arrangements, that in specific situations in combination with adoptive time range equalizers they can have an harmful influence due to the generation of echoes which the baseband time range equalizer is unable to eliminate due to its finite length.
The following prior art is of interest:
1. International Conference On Communications, 23-26, June 1985, Chicago, Band 3, Pages 1234-1238, IEEE, New York, US, E., Giorio et al, "Design and Performance of an adaptive IF Equalizer for 140 Mbit/s 16 QAM System".
2. Communication & Transmission, Band 6, Nr. 3, September 1985, Pages 65-82, Issy-les-Moulineaux, FR; G. Bonnerot et al, "Systemes Hertiziens 140 Mbit/s a 4 et 6 GHz STN 65-140-STN 36/40".
3. ICC '86, Nr. 46.5.1-46.5.5, Advanced Adaptive Equalization of Multilevel QAM Digital Radio Systems, Proc. ICC.
4. European Patent E-OS No. 0 179 393
5. Proc. ICC'85 Nr. 39.1.1-39.1.5, Design and Performance of an Adaptive Equalizer for a 140 Mbit/S 16 QAM System).
6. European Patent Application No. 0 092 907.
7. U.S. Pat. No. 4,361,892--Martin
8. Canadian Pat. No. 1,181,817.